There has conventionally been known an inkjet head which causes a heater arranged in the nozzle of a printhead to generate thermal energy, bubbles ink near the heater by using thermal energy, and discharges ink from the nozzle to print. FIG. 11 shows an example of a heater driving circuit in the inkjet head.
To print at a high speed, heaters are desirably concurrently driven as many as possible to simultaneously discharge ink from many nozzles. However, the electric power supply capacity of the electric power supply of a printer apparatus is limited, and a current value which can be supplied at once is limited by, e.g., a voltage drop caused by the resistance of a wiring line extending from the power supply to the heater. For this reason, time divisional driving of driving a plurality of heaters in time division to discharge ink is generally adopted. In time divisional driving, for example, a plurality of heaters are divided into a plurality of blocks (groups) each formed from adjacent heaters, and driving is so time-divided as not to concurrently drive two or more heaters in each block. This can suppress a total current flowing through heaters and eliminate the need to supply large electric power at once. The operation of the driving circuit which executes this heater driving will be explained with reference to FIG. 11.
NMOS transistors 110211 to 1102mx corresponding to respective heaters 110111 to 1101mx are divided into blocks 1 to m which contain the same number of (x) NMOS transistors, as shown in FIG. 11. More specifically, in block 1, a power supply line from a power supply pad 1104 (+) is commonly connected to the heaters 110111 to 11011x, and the NMOS transistors 110211 to 11021x are series-connected to the corresponding heaters 110111 to 11011x between the power supply pad 1104 (+) and ground 1104 (−). When a control signal is supplied from a control circuit 1105 to the gates of the NMOS transistors 110211 to 11021x, the NMOS transistors 110211 to 11021x are turned on to supply a current from the power supply line through corresponding heaters and heat the heaters 110111 to 1101x.
FIG. 12 is a timing chart showing a timing at which a current is sent to drive heaters in each block of the heater driving circuit shown in FIG. 11.
For example, when block 1 in FIG. 11 is exemplified, control signals VG1 to VGx are timing signals for driving the first to xth heaters 110111 to 11011x belonging to block 1. More specifically, VG1 to VGx represent the waveforms of signals input to the control terminals (gates) of the NMOS transistors 110211 to 11021x of block 1. A corresponding NMOS transistor 1102 is turned on for a high-level control signal, and a corresponding NMOS transistor is turned off for a low-level control signal. This also applies to the remaining blocks 2 to m. In FIG. 12, Ih1 to Ihx represent current values flowing through the heaters 110111 to 11011x.
In this manner, heaters in each block are sequentially driven in time division by sending a current. The number of heaters driven in each block by sending a current can always be controlled to one or less, and no large current need be supplied to a heater.
FIG. 13 depicts a view showing an example of the layout of a heater substrate (substrate which constitutes the printhead) on which the heater driving circuit in FIG. 11 is formed. FIG. 13 shows the layout of power supply lines connected from the power supply pads 1104 (+) and (−) to blocks 1 to m shown in FIG. 11.
Power supply lines 13011 to 1301m are individually connected from the power supply pad 1104 (+) to respective blocks 1 to m, and power supply lines 13021 to 1302m are connected from the power supply pad 1104 (+). As described above, by keeping the maximum number of heaters concurrently driven in each block to one or less, a current value flowing through a wiring line divided for each block can always be suppressed to be equal to or smaller than a current flowing through one heater. Even when a plurality of heaters in different blocks are concurrently driven, voltage drop amounts on wiring lines on the heater substrate can be made uniform. At the same time, even when a plurality of heaters are concurrently driven, the amounts of energy applied to respective heaters can be made almost uniform.
Recently, printers require higher speeds and higher precision, and the printhead of the printer integrates a larger number of nozzles at a higher density. In heater driving of the printhead, heaters are required to be simultaneously driven as many as possible at a high speed in terms of the printing speed.
A heater substrate is prepared by forming many heaters and their driving circuit on the same semiconductor substrate. The number of heater substrates formed from one wafer must be increased to reduce the cost, and downsizing of the heater substrate is also demanded.
When, however, the number of concurrently driven heaters is increased, as described above, the heater substrate requires wiring lines corresponding to the number of concurrently driven heaters. As the number of wiring lines increases, the wiring region per wiring line decreases to increase the wiring resistance when the area of the heater substrate is limited. Further, as the number of wiring lines increases, each wiring width decreases, and variations in resistance between wiring lines on the heater substrate increase. This problem occurs also when the heater substrate is downsized, and the wiring resistance and variations in resistance increase. Since heaters and power supply lines are series-connected to the power supply on the heater substrate, as described above, increases in wiring resistance and resistance variations lead to a high regulation of a voltage applied to each heater.
When energy applied to a heater is too small, ink discharge becomes unstable; when the energy is too large, the heater durability degrades. To print with high quality, energy applied to a heater is desirably constant. However, large fluctuations in voltage applied to a heater degrade the heater durability and make ink discharge unstable, as described above.
Since a wiring line outside the heater substrate is common to a plurality of heaters, the voltage drop on the common wiring line changes depending on the number of concurrently driven heaters. In order to make energy applied to each heater constant against variations in voltage drop, energy applied to each heater is adjusted by the voltage application time. However, as the number of concurrently driven heaters increases, the voltage drop becomes larger on the common wiring line. The voltage application time in heater driving becomes longer, making it difficult to drive a heater at a high speed.
Japanese Patent Laid-Open No. 2001-191531 proposes a method which solves such problems caused by variations in energy applied to a heater. FIG. 14 is a circuit diagram showing a heater driving circuit disclosed in Japanese Patent Laid-Open No. 2001-191531. In this arrangement, printing elements (R1 to Rn) are driven by a constant current using constant current sources (Tr14 to Tr(n+13)) and switching elements (Q1 to Qn) which are arranged for the respective printing elements (R1 to Rn). In this case, constant current sources equal in number to printing elements are necessary, the area of the heater substrate becomes much larger than that in a conventional driving method, and the cost of the heater substrate becomes higher. In order to stabilize energy applied to a heater, output currents from a plurality of constant current sources must be uniform. However, as the number of constant current sources increases, output currents from these constant current sources vary much more. It is difficult to reduce variations in output current between a plurality of constant current sources particularly on a heater substrate having a larger number of heaters for higher speed and higher precision of printing by the printer.